High-brightness, high-resolution cathode-ray tubes are employed in a variety of applications including, for example, monochrome displays used in medical imaging and field-sequential liquid crystal color displays of the type described in U.S. Pat. No. 4,582,396 to Bos et al. These displays are desirable because they are capable of providing high-resolution images in accordance with the resolution of the cathode-ray tube. However, the cathode-ray tube must be capable of forming relatively bright images.
More specifically, field-sequential color displays typically employ a cathode-ray tube with a single electron gun that generates in sequence three color component images that are transmitted through a liquid crystal light shutter to form a full-color display. The liquid crystal light shutters through which the images are transmitted typically attenuate the brightness of the light generated by the cathode-ray tube by between about 93 percent and 95 percent. Similarly, high resolution monochrome displays typically include a high contrast notched or neutral density filter that attenuates the brightness of the light as much as 81 percent. The cathode-ray tube must, therefore, generate light of sufficient brightness to form an acceptable resulting high resolution image.
Many high resolution displays have relatively small display screens with diagonal dimensions of less than about 25.4 cm (10 inches). Such a display screen typically includes no more than about 0.3 million picture elements or pixels and requires a video signal bandwidth of about 30 MHz for monochrome displays and 85 MHz for field-sequential color displays. A video signal bandwidth of about 110 MHz is the maximum at which most cathode-ray tube electron guns can modulate an electron beam.
One limiting factor on the video signal bandwidth capability of most cathode-ray tubes is that the cathode and immediately adjacent control grid electrode that cooperate to modulate the electron beam each have a relatively high capacitance. An electron gun having a low capacitance cathode and a low capacitance control grid electrode is described in U.S. Pat. No. 4,500,809 of Odenthal et al.
This electron gun includes an accelerating grid electrode that is configured as the frustum of a cone and, therefore, has a tendency to magnify the spherical aberration of the electron beam. The effect of spherical aberration is particularly disadvantageous in high resolution displays because it enlarges the electron beam spot size characteristic, consequently interfering with the high resolution performance of the cathode-ray tube. An electron gun of this type has inadequate performance characteristics for high-resolution, high-brightness display images.
U.S. Pat. No. 5,077,498 of Odenthal describes a cathode-ray tube with a flat, relatively thick accelerating grid electrode that cooperates with a high capacitance cap-shaped control grid electrode to provide an electron beam with reduced spherical aberration. The accelerating grid electrode is referred to as thick because its thickness is approximately equal to the diameter of the aperture in the electrode. Although an electron gun of this type is adequate for some high-brightness, high-resolution display applications, the high capacitance characteristics of the control grid electrode limit the video signal bandwidth capability of the electron gun. As a consequence, this electron gun is incapable of providing very high resolution, high-brightness display images on a large display screen.